1. Field of the Invention
The present invention relates to a plate for use as a light shielding blade that is required to move at a high speed, such as shutter blades of a focal plane shutter or a lens shutter of a camera, or diaphragm blades thereof.
2. Related Background Art
For such light shielding blades, there are required a small actuating force and a sufficient bending rigidity, in addition to the ability for high-speed movement.
For meeting these requirements, there is required a light shielding blade of light weight, high strength and high elasticity.
The present inventor has investigated a plastic sheet reinforced with continuous carbon fibers oriented in a predetermined direction (hereinafter referred to as CFRP (carbon fiber reinforced plastic) sheet), and identified that a plate material obtained by laminating a plurality of such sheets with mutually orthogonal or substantially orthogonal fiber directions was suitable as the material for light shielding blades capable of high speed movement in view of weight, strength and ease of manufacture (cf. Japanese Laid-open Pat. No. Sho 59-61827).
In this case, said plate material is produced by laminating a plurality of prepreg sheets which are precursors of CFRP sheets in such a manner that the directions of fibers mutually cross orthogonally or substantially orthogonally, and hardening the entire laminate by heating in a pressed state.
Said prepreg sheet is obtained by aligning continuous carbon fibers in a direction, in the form of a thin plate, impregnating said carbon fibers with thermosetting resin liquid which is a precursor of matrix resin (for example unhardened liquid of epoxy resin or unsaturated polyester) and transforming said resin liquid into so-called B-stage, in which the resin is already solidified and lacks fluidity but can be finally hardened by heating. It has a low resin content, and, in appearance, it looks like a bundle of hair-like carbon fibers each thinly coated with glue-like resin liquid.
However, such plate material is almost unable to provide complete light shielding property only by the carbon fibers because of its small thickness, and is always associated with translucent portions scattered like pinholes. This fact leads to a high wastage rate in the cutting for preparing light shielding blades, and thus to a higher production cost of such blades.
For this reason it is an attractive proposal to mix black pigment (carbon black) in the matrix resin of the CFRP sheet, in addition to the carbon fibers, thereby filling the gaps of the carbon fibers with said pigment and thus improving the light shielding property.
However, the plate material obtained by laminating the sheets with black pigment dispersed in the matrix resin has been associated with poor flatness.
Also such plate material has often shown microcracks and cavities, giving rise to local loss of strength and having often resulted in defects in the connecting part with a driving system. Also in appearance, there have been observed formation of microcracks and striped patterns.
The material with such appearance has been associated with poor receptivity for black lubricating coating (hereinafter called DL coating) to be applied in a later stage for the purpose of improving abrasion resistance, scratch resistance, lubricating ability, light shielding ability and appearance.